The purpose of the intake of civil turbofan gas turbine engines is to ensure that, under all operating conditions, the engine is supplied with the correct quantity of air and that the air has sufficient flow uniformity to allow efficient and stable operation of the engine. The intake design is integrated into the engine nacelle to obtain the lowest level of drag at the operating design point, typically cruise, for maximum efficiency.
For civil turbofans, an optimum intake configuration is a short, near circular, pilot-type intake. This design is highly efficient for subsonic operation, as low levels of pressure loss are achieved under most and possibly all operating conditions.
A pilot intake consists of two geometric regions, a lip and a throat. Downstream of the throat the airflow passes into a diffuser where the flow area is increased up to a fan entry plane. The diffuser acts as a settling length to improve the uniformity of the airflow entering the fan. The lip is a forward section and similar in section to an aerofoil and is shaped to guide the airflow into the engine under all operating conditions; the lip is also optimised to prevent flow separation under cross-wind and incidence operation. If flow separation occurs, this produces significant asymmetry in the total pressure within the intake increasing fan blade stresses and in severe cases engine surge.
The internal surface of the lip is heated, usually with hot air, to minimise or ensure that there is no ice accretion, which could otherwise be shed and damage the engine. Other heating arrangements include electrical heating pads.
The diffuser and throat regions of the intake are often lined with sound-absorbing acoustic panels to reduce noise from the fan and other turbomachinery of the engine.
It is desirable to provide an intake lining system which provides improved flow control to suppress shock waves and/or separation of the airflow from the intake's internal surface that can be caused by cross winds and high incident conditions.
It is also desirable to provide anti-icing and acoustic treatments that minimise weight, complexity and parts count.